Friction and its types



Rolling Friction

A solid sphere of mass is placed on a rough inclined plane as shown in the figure. The coefficient is insufficient to start pure rolling. The sphere slides a length on the incline from rest and its kinetic energy becomes . Then, the work done by friction will be given by:
Work done by friction + Work done by gravity + work done by normal force = Kinetic energy
(Work energy theorem)
work done by normal force=0
Work done by gravity is mglsin
Work done by friction =


Static Friction

Static friction is the force which acts between two bodies when a force is applied on a body but there is no relative motion between the surfaces. Static friction opposes an impending motion. It is denoted by . Law of static friction is given by:

where is the coefficient of static friction and depends only on the nature of the surfaces in contact. is the normal force.


Sliding Friction

The Sliding Friction is friction which comes into play when surface of one object moves relative to surface of another object. It is also called as kinetic friction. It is that force that keeps a surface sliding with another surface. For example, if book is kept on floor and the book moves with respect to the floor.

It is the frictional force between two surfaces that are rubbing against each other. It’s hard to find a perfectly smooth surface in the real life, therefore when an object slides on any surface, it undergoes a backward force because of the relative motion between the two adjacent surfaces. It always acts against the motion.